1. Field of the Invention
This invention relates generally to polyimides. It relates particularly to soluble, thermally-processable polyimides with high glass transition temperatures and good mechanical properties. These polyimides can crosslink either thermally or photolytically to yield insoluble polymer networks. It also relates to novel monomers used in the preparation of these polyimides.
2. Description of the Prior Art
Polyimides are condensation polymers commonly synthesized by the reaction of aromatic dianhydrides with aromatic diamines. The intermediate polyamide acid is either thermally or chemically cyclodehydrated to form the polyimide as depicted in Equation (1). ##STR1## Ar is a tetravalent aromatic radical which can be as simple as 1,2,4,5-tetrasubstituted benzene. Ar may be a bis-4-(o-diphenylene) having the generic structure ##STR2## where X=nil, O, S, SO.sub.2, C.dbd.O, Si(CH.sub.3).sub.2, and the like, or Ar may be any other appropriate tetravalent radical. Ar' is a divalent aromatic radical which may be 1,3-phenylene, 1,4-phenylene, 4,4'-biphenylene, 4,4'-oxydiphenylene, 4,4'-thiodiphenylene, 4,4'-carbonyldiphenylene, 4,4'-methanediphenylene, or any other appropriate divalent radical.
Synthesis and characterization of polyimides has been extensively reported in the literature. The preparation of aromatic polyimides by reaction of an aromatic dianhydride with an aromatic diamine, followed by thermal cyclization was first reported in 1963 [G. M. Bower and L. W. Frost, J. of Polym. Sci., A1, 3135 (1963)]. Several reviews on polyimides have been published [C. E. Sroog, "Polyimides" in Encyclopedia of Polym. Sci. and Technology, (H. R. Mark, N. G. Gaylord, and N. M. Bikales, Ed.), Interscience Publishers New York, 1969, Vol. 11, pp. 247-272; N. A. Adrova, M. I. Bessonov, L. A. Lauis, and A. P. Rudakov, Polyimides, Technomic Publishing Co., Inc., Stamford, Conn., 1970]; D. Wilson, H. D. Stenzenberger and P. M. Hergenrother, Polyimides, Blackie and Son Ltd., Glasgow, United Kingdom, 1990. Wholly aromatic polyimides are known for their exceptional thermal, thermooxidative and chemical resistance.
Typically the polyamide acid precursors are soluble in the reaction medium, a polar aprotic solvent such as N,N-dimethylacetamide, N,N-dimethylformamide, N-methylpyrrolidinone, dimethyl sulfoxide, or ether solvents such as diglyme. Upon ring closure the resulting polyimide usually becomes insoluble in the previously mentioned solvents. Consequently, all solution processing, for example, prepregging, film casting, fiber spinning, and coating application, is normally done with the polyamide acid. Soluble high molecular weight polyimides are rare and limited to cases where solubilizing groups are incorporated to reduce chain regularity or where the effect of the aromatic polyimide moiety is diluted by the presence of more flexible groups. For example, XU-218, an amorphous polyimide commercially available from Ciba-Geigy, is soluble in N,N-dimethylacetamide due to incorporation of an asymmetric 1,3,3-trimethyl-1-phenylindene group into the polyimide backbone. The incorporation of oxyethylene units into polyimide backbones has resulted in material which are soluble in N-methylpyrrolidinone [F. W. Harris, A. J. Karnavas, S. Das, C. N. Curcuras, and P. M. Hergenrother, Polymeric Materials: Science and Engineering Preprints, 54, 89 (1986)].
The addition of methyl groups onto an aromatic ring of a polyimide chain has been found in some instances to be an effective means of disrupting the regularity of the chain and increasing the glass transition temperature of the polyimide [F. W. Harris and S. H. S. Lien, Polymeric Materials: Science and Engineering Preprints, 60, 197 (1989)].
A primary object of the present invention is to provide new polyimides that are especially useful as coatings, films, membranes, photoresists, and insulators.
Another object of the present invention is to provide soluble, thermally processable polyimides with high glass transition temperatures and good mechanical properties which can crosslink to yield insoluble polymer networks.
Another object of the present invention is the synthesis of two new monomers which have utility in the production of the above polyimides: 1,3-bis(3-methyl-4-aminophenoxy-4'-benzoyl)benzene and 1,3-bis(3,5-dimethyl-4-aminophenoxy-4'-benzoyl)benzene.